1. Field of the Invention
This invention relates generally to the field of raster correction circuits, and, in particular, to correction of orthogonality and parallelogram errors in a raster of a cathode ray tube of a video display apparatus.
2. Description of Prior Art
In a cathode ray tube (CRT) of a video display apparatus, a raster is formed by deflecting at least one electron beam across a phosphor screen. Each electron beam is deflected in a horizontal direction by a magnetic field produced by the excitation of a horizontal deflection coil by a horizontal-rate sawtooth current. Likewise, each electron beam is simultaneously deflected in a vertical direction by a magnetic field produced by the excitation of a vertical deflection coil by a vertical-rate sawtooth current. The result is a negatively-sloped, or "downhill", scan line as the electron beam is deflected from left to right to form the CRT's raster. In a typical cathode ray tube used in a color television receiver and having a screen width of approximately 723 mm and a screen height of approximately 538 mm, a horizontal scan line may drop a distance of approximately 2.4 mm from a perfectly horizontal position in one field.
This downhill scan effect introduces both orthogonality and parallelogram errors into the raster, as shown in FIG. 1. In a perfectly rectangular raster, horizontal and vertical center lines are orthogonal, or perpendicular, to one another. Downhill scanning does not produce a perfectly rectangular raster and hence results in a non-orthogonal relationship between the horizontal and vertical center lines of the raster.
Orthogonality error is a quantitative measure, expressed in units of radians or degrees, of the extent to which the horizontal and vertical center lines of a raster depart from orthogonality. For a raster represented in terms of X and Y co-ordinates, as depicted in FIG. 2, the orthogonality error can be calculated with the following trigonometric formula: ##EQU1## A conventional downhill scan may produce an orthogonality error on the order of approximately 0.2.degree.. A typical design tolerance for the orthogonality error in a CRT may be specified as .+-.0.3.degree..
The orthogonality error may be magnified at the left and right edges of the raster because, as is well-known, the deflection sensitivity of an electron beam increases as it approaches the edges of the raster. As a result, the edges of the raster may tilt such that the raster has a generally parallelogram shape.
Parallelogram error is a quantitative measure, expressed in units of radians or degrees, of the extent to which the shape of a raster approximates a parallelogram. For a raster represented in terms of X and Y co-ordinates, as depicted in FIG. 2, the vertical parallelogram error can be calculated with the following trigonometric formula: ##EQU2## The horizontal parallelogram error can be calculated with the following trigonometric formula: ##EQU3## In a conventional downhill scan, a typical orthogonality error may translate into a parallelogram error that is on the order of approximately 1.5 times the orthogonality error. For example, a conventional downhill scan that produces an orthogonality error of 0.2.degree. may also produce a parallelogram error that is equal to approximately 0.3.degree.. A typical design tolerance for the parallelogram error in a CRT may be specified as +0.5.degree..
If means are employed to correct side, or east-west, pincushion distortion in a raster, the downhill scan effect may cause a misalignment of a pincushion correction current envelope with respect to the pincushion curvature on the raster. Mitigation of this misalignment may result in an increase in the parallelogram error by an amount that may equal approximately 80%. Thus, for a conventional downhill scan that produces a parallelogram error equal to approximately 0.3.degree., the use of side pincushion correction may increase the parallelogram error to approximately 0.6.degree..
It is desirable to completely eliminate both orthogonality and parallelogram errors in a raster so that a CRT may display the highest-quality image. One possible solution requires rotation of the horizontal deflection coil relative to the vertical deflection coil in order to align the sloped horizontal center line of the raster with the horizontal center line of the CRT. The downhill scan effect is thereby eliminated, but this approach can, nonetheless, be problematic. First, this solution can affect convergence in the video display apparatus. Second, as the sloped horizontal center line is rotated toward the center line of the CRT, the pincushion curvature on the raster also rotates in order to maintain its original relationship with the sloped horizontal center line. Thus, while this solution can eliminate the orthogonality error, it does not address the component of the parallelogram error due to misalignment of the pincushion correction current envelope with respect to the pincushion curvature on the raster.